Semiconductor nanoparticle-based light emitting materials

ABSTRACT

A light emitting layer including a plurality of light emitting particles embedded within a host matrix material. Each of said light emitting particles includes a population of semiconductor nanoparticles embedded within a polymeric encapsulation medium. A method of fabricating a light emitting layer comprising a plurality of light emitting particles embedded within a host matrix material, each of said light emitting particles comprising a population of semiconductor nanoparticles embedded within a polymeric encapsulation medium. The method comprises providing a dispersion containing said light emitting particles, depositing said dispersion to form a film, and processing said film to produce said light emitting layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/424,556 filed on Feb. 3, 2017 which is a division of U.S. applicationSer. No. 13/624,632 filed on Sep. 21, 2012, and issued as U.S. Pat. No.10,217,908 which claims priority to U.S. Provisional Application No.61/538,301 filed Sep. 23, 2011, and to Great Britain Application No.1116517.2 filed Sep. 23, 2011, the contents of which are herebyincorporated by reference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

FIELD OF THE INVENTION

The present invention relates to semiconductor-based light emittinglayers and devices incorporating such layers. The present invention alsorelates to methods of fabricating such layers.

BACKGROUND OF THE INVENTION

Conventional backlight units have consisted of a cold cathodefluorescent lamp (CCFL) and a diffuser sheet to give large areas ofhomogenous white light. Due to energy and size constraints more recentlyRGB-LEDs have replaced the CCFL light source (FIG. 1). A furtherdevelopment has been to use a blue LED excitation source in combinationwith a sheet containing a conventional phosphor, such as YAG, wherebythe “phosphor layer” or “phosphor sheet” is located near or on top ofthe diffuser layer and away from the light/excitation source (FIG. 2).

Currently phosphorescent materials used in down converting applications,absorb UV or mainly blue light and convert it to longer wavelengths,with most phosphors currently using trivalent rare-earth doped oxides orhalophosphates. White emission is obtained by blending phosphors whichemit in the blue, green and red regions with that of a blue or UVemitting solid-state device. i.e. a blue light emitting LED plus a greenphosphor such as, SrGa2S4:Eu²⁺, and a red phosphor such as, SrSiEu²⁺ ora UV light emitting LED plus a yellow phosphor such as,Sr₂P₂O₇:Eu²⁺;Mn²⁺, and a blue-green phosphor.

Presently white LEDs are made by combining a blue LED with a yellowphosphor however, color control and color rendering is poor when usingthis methodology due to lack of tunability of the LEDs and the phosphor.Moreover, conventional LED phosphor technology uses down convertingmaterials that have poor color rendering (i.e. color rendering index(CRI) <75) due to the lack of available phosphor colors.

There has been substantial interest in exploiting the properties ofcompound semiconductors consisting of particles with dimensions in theorder of 2-50 nm, often referred to as Quantum Dots (QDs) ornanocrystals. These materials are of commercial interest due to theirsize-tunable electronic properties which can be exploited in manycommercial applications such as optical and electronic devices and otherapplications that now range from biological labeling, photovoltaics,catalysis, light-emitting diodes, general space lighting andelectroluminescent displays amongst many new and emerging applications.Two fundamental factors, both related to the size of the individualsemiconductor nanoparticle, are responsible for their unique properties.The first is the large surface to volume ratio; as a particle becomessmaller, the ratio of the number of surface atoms to those in theinterior increases. This leads to the surface properties playing animportant role in the overall properties of the material. The secondfactor, which affects many materials including semiconductornanoparticles, is a change in the electronic properties of the materialswith size; because of quantum confinement effects the band gap graduallybecomes larger as the size of the particle decreases. This effect is aconsequence of the confinement of an ‘electron in a box’ giving rise todiscrete energy levels similar to those observed in atoms and molecules,rather than a continuous band as observed in the corresponding bulksemiconductor material. Thus, for a semiconductor nanoparticle, becauseof the physical parameters, the “electron and hole”, produced by theabsorption of electromagnetic radiation, a photon, with energy greaterthan the first excitonic transition, are closer together than they wouldbe in the corresponding macrocrystalline material; moreover theCoulombic interaction cannot be neglected. This leads to a narrowbandwidth emission, which is dependent upon the particle size andcomposition of the nanoparticle material. Thus, QDs have higher kineticenergy than the corresponding macrocrystalline material and consequentlythe first excitonic transition (band gap) increases in energy withdecreasing particle diameter.

Core semiconductor nanoparticles, which consist of a singlesemiconductor material along with an outer organic passivating layer,tend to have relatively low quantum efficiencies due to electron-holerecombination occurring at defects and dangling bonds situated on thenanoparticle surface which can lead to non-radiative electron-holerecombinations.

One method to eliminate defects and dangling bonds on the inorganicsurface of the QD is to overcoat the nanoparticles with a homogeneousshell of a second semiconductor. This semiconductor material typicallyhas a much wider band-gap than that of the core to suppress tunneling ofthe charge carriers from the core to the newly formed surface atoms ofthe shell. The shell material must also have a small lattice mismatch tothat of the core material. Lattice mismatch arises primarily because ofthe differences in bond lengths between the atoms in the core and in theshell. Although the differences in the lattice mismatch between the coreand shell materials may only be a few percent it is enough to alter boththe kinetics of shell deposition and particle morphology as well as theQY of the resultant particles. Small lattice mismatch is essential toensure epitaxial growth of the shell on the surface of the core particleto produce a “core-shell” particle with no or minimum defects at theinterface that could introduce non-radiative recombination pathways thatreduce the PLQY of the particle. One example is a ZnS shell grown on thesurface of a CdSe or InP core. The lattice mismatch of some of the mostcommon shell materials relative to CdSe is 3.86% for CdS, 6.98% for ZnSeand 11.2% for ZnS.

Another approach is to prepare a core-multi shell structure where the“electron-hole” pair is completely confined to a single shell layerconsisting of a few monolayers of a specific material such as aQD-quantum well structure. Here, the core is of a wide band gapmaterial, followed by a thin shell of narrower band gap material, andcapped with a further wide band gap layer, such as CdS/HgS/CdS grownusing substitution of Hg for Cd on the surface of the core nanocrystalto deposit just a few monolayers of HgS which is then over grown by amonolayer of CdS. The resulting structures exhibit clear confinement ofphotoexcited carriers in the HgS layer, which result in a high PLQY andimproved photochemical stability.

To add further stability to QDs and help to confine the electron-holepair one of the most common approaches is to grow thick and robust shelllayers around the core. However, because of the lattice mismatch betweenthe core and shell materials, the interface strain accumulatesdramatically with increasing shell thickness, and eventually can bereleased through the formation of misfit dislocations, degrading theoptical properties of the QDs. This problem can be circumvented byepitaxially growing a compositionally graded alloy layer on the core asthis can help to alleviate the strain at the core-shell interface. Forexample in order to improve the structural stability and quantum yieldof a CdSe core, a graded alloy layer of Cd1-xZnxSe1-ySy can be used inplace of a shell of ZnS directly on the core. Because of the gradualchange in shell composition and lattice parameters the resulting gradedmulti-shell QDs are very well electronically passivated with PLQY valuesin the range of 70-80% and present enhanced photochemical and colloidalstability compared to simple core-shell QDs.

Doping QDs with atomic impurities is an efficient way also ofmanipulating the emission and absorption properties of the nanoparticle.Procedures for doping wide band gap materials, such as zinc selenide andzinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), have beendeveloped. Doping with different luminescence activators in asemiconducting nanocrystal can tune the photoluminescence andelectroluminescence at energies even lower than the band gap of the bulkmaterial, whereas the quantum size effect can tune the excitation energywith the size of the QDs without having a significant change in theenergy of the activator related emission. Dopants include main group orrare earth elements, often a transition metal or rare earth element,such as, Mn⁺² or Cu²⁺.

The coordination around the atoms on the surface of any core, core-shellor core-multi shell, doped or graded nanoparticle is incomplete and thenon-fully coordinated atoms have dangling bonds which make them highlyreactive and can lead to particle agglomeration. This problem isovercome by passivating (capping) the “bare” surface atoms withprotecting organic groups.

The use of QDs in light emitting devices has some significant advantagesover the use of the more conventional phosphors such as the ability totune the emission wavelength, strong absorption properties and lowscattering if the QDs are mono-dispersed. However, the methods used sofar are challenging due to chemical incompatibility between the outerorganic surfaces of the QDs and the types of host materials in which theQDs are supported. QDs can suffer from agglomeration when formulatinginto these materials and, once incorporated, can suffer fromphoto-oxidation as a result of the migration of oxygen through the hostmaterial to the surfaces of the QDs, which can ultimately lead to a dropin quantum yield. Although reasonable devices can be made underlaboratory conditions, there remain significant challenges to replicatethis under commercial conditions on a large scale. For example, at themixing stage the QDs need to be stable to air.

Devices incorporating a light emitting layer where semiconductor QDs areused in place of the conventional phosphors have been described,however, due to problems relating to processability and the stability ofthe QD-containing materials during and after layer fabrication, the onlytypes of QD material that have been successfully incorporated into suchlayers are relatively conventional II-VI or IV-VI QD materials, e.g.CdSe, CdS and PbSe. Cadmium and other restricted heavy metals used inconventional QDs are highly toxic elements and represent a major concernin commercial applications. The inherent toxicity of cadmium-containingQDs prevents their use in any applications involving animals or humans.For example, recent studies suggest that QDs made of a cadmiumchalcogenide semiconductor material can be cytotoxic in a biologicalenvironment unless protected. Specifically, oxidation or chemical attackthrough a variety of pathways can lead to the formation of cadmium ionson the QD surface that can be released into the surrounding environment.Although surface coatings such as ZnS can significantly reduce thetoxicity, it may not completely eliminate it because QDs can be retainedin cells or accumulated in the body for a long period of time, duringwhich their coatings may undergo some sort of degradation exposing thecadmium-rich core.

The toxicity affects not only the progress of biological applicationsbut also other applications including optoelectronic and communicationbecause heavy metal-based materials are widespread in many commercialproducts including household appliances such as IT & telecommunicationequipment, lighting equipment, electrical & electronic tools, toys,leisure & sports equipment. A legislation to restrict or ban certainheavy metals in commercial products has been already implemented in manyregions of the world. For example, starting 1 Jul. 2006, the EuropeanUnion directive 2002/95/EC, known as the “Restrictions on the use ofHazardous Substances in electronic equipment” (or RoHS), banned the saleof new electrical and electronic equipment containing more than agreedlevels of lead, cadmium, mercury, hexavalent chromium along withpolybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE)flame retardants. This law required manufacturers to find alternativematerials and develop new engineering processes for the creation ofcommon electronic equipment. In addition, on 1 Jun. 2007 a EuropeanCommunity Regulation came into force concerning chemicals and their safeuse (EC 1907/2006). The Regulation deals with the Registration,Evaluation, Authorisation and Restriction of Chemical substances and isknown as “REACH”. The REACH Regulation gives greater responsibility toindustry to manage the risks from chemicals and to provide safetyinformation on the substances. It is anticipated that similarregulations will be extended worldwide including China, Korea, Japan andthe US.

There are currently no light emitting layers available that containheavy metal-free QDs, which can be fabricated at commercially feasiblecost and that emit light efficiently in the visible spectrum.

SUMMARY

The disclosure provides light emitting materials and/or methods offabricating such materials which contain heavy metal-free QDs.

The disclosure provides emitting materials and/or methods of fabricatingsuch materials which can be fabricated at commercially feasible cost.

The disclosure provides light emitting materials and/or methods offabricating such materials that emit light efficiently in the visiblespectrum.

The disclosure provides formulations containing QDs that can be used tofabricate light emitting materials and/or methods of fabricating suchmaterials using said formulations.

The disclosure obviates or mitigates one or more of the problemsassociated with current light emitting materials and/or methods offabricating such materials.

According to a first aspect of the disclosure, there is provided a lightemitting layer comprising a plurality of light emitting particlesembedded within a host matrix material, each of said light emittingparticles comprising a population of semiconductor nanoparticlesembedded within a polymeric encapsulation medium.

A second aspect of the disclosure provides a method of fabricating alight emitting layer comprising a plurality of light emitting particlesembedded within a host matrix material, each of said light emittingparticles comprising a population of semiconductor nanoparticlesembedded within a polymeric encapsulation medium, the method comprisingproviding a dispersion containing said light emitting particles,depositing said dispersion to form a film, and processing said film toproduce said light emitting layer.

A third aspect of the disclosure provides a light emitting devicecomprising a light emitting layer in optical communication with a lightdiffusion layer, said light emitting layer comprising a plurality oflight emitting particles embedded within a host matrix material, each ofsaid light emitting particles comprising a population of semiconductornanoparticles embedded within a polymeric encapsulation medium.

According to a fourth aspect of the disclosure, there is provided alight emitting device comprising a light emitting layer in opticalcommunication with a backlight, said light emitting layer comprising aplurality of light emitting particles embedded within a host matrixmaterial, each of said light emitting particles comprising a populationof semiconductor nanoparticles embedded within a polymeric encapsulationmedium.

According to a fifth aspect of the disclosure there is provided adispersion suitable for printing or drop casting on to a substrate, thedispersion comprising light emitting particles dispersed in a hostmatrix material, each of said light emitting particles comprising apopulation of semiconductor nanoparticles embedded within a polymericencapsulation medium.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, includingfeatures and advantages, reference is now made to the detaileddescription of the invention along with the accompanying figures:

FIG. 1 is a schematic illustration of a prior art back light unitincorporating conventional LEDs;

FIG. 2 is a schematic illustration of a prior art back light unitincorporating a light emitting layer, often referred to as a ‘phosphorsheet’;

FIG. 3 is a schematic illustration of a first embodiment of a back lightunit incorporating a QD-bead phosphor sheet combining a layer of greenQD-beads and a layer of red QD-beads in accordance with a preferredembodiment of the present disclosure;

FIG. 4 is a schematic illustration of a second embodiment of a backlight unit incorporating a QD-bead phosphor sheet combining green andred QD-beads in the same layer in accordance with another preferredembodiment of the present disclosure;

FIG. 5 is a schematic illustration of how multi-colored (in this case,red and green) QDs can be combined within the same bead such that eachbead emits white secondary light when illuminated by a primary lightsource (in this case a blue light source);

FIG. 6 is a schematic illustration of how QDs of a single color (in thiscase, red, green or blue) can be encapsulated within beads and then thedifferently colored QD-beads combined within a device such that thedevice emits white secondary light when illuminated by a primary lightsource (in this case a UV light source);

FIG. 7 is a schematic illustration of how QDs of a single color (in thiscase, red) can be encapsulated within beads and then incorporated into adevice such that the device emits secondary light of the same color asthe QDs when illuminated by a primary light source (in this case an LEDchip);

FIG. 8 shows the UV-vis absorption and PL spectra of CdSe/ZnS core/shellQDs in toluene used in the formulation of inks according to aspects ofthe present disclosure and in the fabrication of phosphor sheets inaccordance with other aspects of the present disclosure; and

FIG. 9 shows the UV-vis absorption and PL spectra of InP/ZnS core/shellQDs in toluene used in the formulation of inks according to aspects ofthe present disclosure and in the fabrication of phosphor sheets inaccordance with other aspects of the present disclosure.

DETAILED DESCRIPTION

The introduction of semiconductor QDs into the emitting material inaccordance with the disclosure brings several advantages. High luminousefficiency can be achieved with a UV light source exciting the QDs whichremoves the need of filters, hence reducing the loss of light intensity.The color range attainable in the device is enhanced and can begradually tuned by varying the size or the composition of the QDs, forexample, a range of colors can be obtained from blue to deep red to spanthe entire visible spectrum by varying the size of CdSe or InP QDs. Thesize of InAs and PbSe QDs can be tuned to cover most of the near- andmiddle-infrared regions. QD displays yield more purity in colors thanother types of display technologies because QDs exhibit very narrowemission bandwidths and can create pure blue, green, and red to generateall other colors with the results of an improved viewing experience forthe end user. By tailoring their synthesis, the QDs can be easilydispersed into aqueous or organic mediums enabling fast and economicdevice manufacturing with standard printing or othersolution-processable techniques; this also provides an opportunity tocreate printable and flexible devices. There is an increasing interestin the development of flexible emitting substrates to meet the growingdemand for low-cost, large-area, flexible and lightweight devices, suchas roll-up displays, e-papers, and keyboards.

The semiconductor nanoparticles preferably contain ions selected fromgroup 11, 12, 13, 14, 15 and/or 16 of the periodic table, or saidquantum dots contain one or more types of transition metal ion ord-block metal ion. The semiconductor nanoparticles may contain one ormore semiconductor material selected from the group consisting of CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN,GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, MgTe and combinationsthereof.

The polymeric encapsulation medium is preferably an opticallytransparent medium comprising a material selected from the groupconsisting of a polymer, a resin, a monolith, a glass, a sol gel, anepoxy, a silicone and a (meth)acrylate. The polymeric encapsulationmedium may comprise a material selected from the group consisting ofpoly(methyl (meth)acrylate), poly(ethylene glycol dimethacrylate),polyvinyl acetate), poly(divinyl benzene), poly(thioether), silica,polyepoxide and combinations thereof.

The light emitting particles are preferably discrete microbeads, eachmicrobead incorporating a plurality of said semiconductor nanoparticles.Said microbeads may possess an average diameter of around 20 nm toaround 0.5 mm. Some or all of the nanoparticle containing microbeads mayinclude a core comprising a first optically transparent medium and oneor more outer layers of the same or one or more different opticallytransparent media deposited on said core. The semiconductornanoparticles may be confined to the core of the microbeads or may bedispersed throughout the core and/or one or more of the outer layers ofthe microbeads.

In the light emitting layer the host matrix material may be selectedfrom a wide variety of polymers whether organic or inorganic, glass,water soluble or organic solvent soluble, biological or synthetic. Forexample, the following simple linear chain polymers may be usedpolyacrylate, polycarbonate, polystyrene, polyethylene, polypropylene,poly ketone, polyether ether ketone, polyesters, polyimide, polyimide,polyacrylamide, polyolefines, polyacetylene, polyisoprene,polybutadiene, PVDF, PVC, EVA, PET, polyurethane, cellulose polymers(e.g., ethylcellulose, isopropylmethylcellulose phthalate,nitrocellulose). Further examples include crosslinked polymers and/orcopolymers, triblock copolymers and UV- and thermal curing epoxy.Suitable polymers may be selected from the group consisting ofpolystyrene/toluene matrix, trimethylol propane trimethacrylate/laurylmethacrylate matrix, trimethylol propane trimethacrylate/laurylmethacrylate/polyisobutylene matrix, trimethylol propanetrimethacrylate/lauryl methacrylate/PIPS matrix, isobornylacrylate/dipropyleneglycol diacrylate matrix,acrylic-polystyrene/toluene matrix, and polycarbonate. Clay materialssuch as bentonite, kaolin, fumed silica (e.g. CabO-Sil™), fumed alumina,fumed zinc oxide, inorganic polymers can be used as the host matrixmedium alone or as additives to organic polymers in order to improve theperformance of the final material. The method according to the presentdisclosure may employ any of the polymers and materials indicated abovealone or in combination with one or more other suitable polymers andmaterials.

In the method of fabricating a light emitting layer the dispersionpreferably possesses a viscosity that makes it suitable for depositionby printing or drop casting. Deposition of said dispersion is preferablyeffected by printing or drop casting. The deposited film may then beprocessing by doctor blading to form a thin film of consistent thicknessover a surface of a substrate. The film may be formed having anydesirable thickness but is preferably up to around 250 nm thick.Processing of the film may also comprise annealing, which may involveheating the film one or more times, for example up to a temperature ofaround 50 to 100° C. In some instances, processing of the film maycomprise annealing, which may involve heating the film one or moretimes, for example up to a temperature of around 300° C. Alternativelyor additionally, processing of the film may comprise curing by anyconvenient means.

In the light emitting device comprising a light emitting layer inoptical communication with a backlight, it is preferred that the devicecomprises a light diffusion layer in between said backlight and saidlight emitting layer.

Fluorescent inks, i.e., inks that emit light under UV or visibleirradiation, have been used for a long time in consumer products for avariety of purposes. One of the main reasons is that fluorescent inksproduce very bright and saturated colors which can make the product moreappealing to the human eye. Many conventional luminescent inks are madeby mixing a transparent base ink with various types of fluorescentpigments. Although these pigments can provide the desired degree ofluminescence, in many cases due to their ability to scatter the lightthey can make the ink opaque which is often an undesirable side effect.Opacity becomes an issue when high loadings of pigments are necessary toachieve the desired brightness or when the ink is used as a primary inkto be combined by overprinting to create secondary and tertiary colors.For example, a transparent blue ink that is overprinted on top of ayellow transparent ink will results into a green ink. On the contrary,an opaque blue ink overprinted on top of another ink will hide theunderlying ink independently of its color and the final ink willcontinue to appear blue to the viewer because of its opacity.

Other than for aesthetic purposes, the need for transparent inks canalso be appreciated in the case of ultraviolet luminescent inks that aremuch sought-after in the manufacture of security articles, such aspassports, personal identification cards, credit cards, chip-and-pincards, bank notes and barcode-tracked products. The main purpose ofthese inks is to introduce one or more distinctive ‘secret’ codes intothe articles in order to make them unique and difficult to counterfeit.The ink must be transparent under the natural light in order to beconcealed and becomes visible only as it emits a certain light colorupon UV irradiation. Ideally the color of the emitted light can be tunedso that it can be recognized only by a specific electronic deviceoverall making the article less prone to forgery and alteration. Thecolor of the emitted light is not necessarily restricted to the visiblerange and can include also light emitted in the infrared portion of thespectrum. The conventional phosphor powders currently used in mostsecurity luminescent inks have an appreciable particle size (usually inthe range of few microns) which causes scattering of the visible lightand makes the inks opaque.

Other conventional luminescent inks are made by mixing a transparentbase ink with various types of organic fluorescent dyes. These types ofinks usually offer high brightness and high transparency but typicallysuffer from low light and water fastness (i.e., the degree to which adye resists fading due to light and water exposure), a phenomenon thatusually worsens in presence of oxygen. Examples of these organic dyesinclude xanthene dyes, diphenyl dyes, diphenyl methane dyes, triarylmethane dyes and mixtures thereof. Another important limitation of theorganic dyes is that they are characterized by a broad emission spectrumwhen excited with UV or visible excitation, which limits the number andthe purity of the colors available and therefore offer limitedprotection against counterfeit.

QD-based inks can offer the same level of brightness without thedrawbacks of the conventional pigment- or dye-based inks. The use of QDhas some significant advantages such as the ability to tune the emissionwavelength, strong absorption properties and low scattering if the QDsare mono-dispersed. For QD it has been found that they can emit light inany near monochromatic color, with the color of the light emitted beingdependent only on the size of the QDs. The QDs can be soluble insolvents and their physical properties can be tailored to be soluble inany type of solvent.

For their use in luminescent inks QDs must be incorporated into an inkmedium while remaining fully mono-dispersed, without significant loss ofquantum efficiency. The methods used so far are challenging due tochemical incompatibility between the outer organic surfaces of the QDsand the medium used in the ink which is preferably water or anaqueous-based solvent. This comes from the fact that the surface of theQDs is typically capped with hydrophobic organic ligands which confervery low or no affinity with water. Hydrophilic ligand-capped QDs havebetter affinity to water-based mediums but often have poorer opticalproperties than their organic equivalents such as low quantum yield andbroad size distribution. Generally, whether they have a hydrophilic- orhydrophobic-surface coating, QDs can still suffer from agglomerationwhen formulating into these inks and once incorporated migration ofoxygen through the ink medium to the surfaces of the QDs can lead tophoto-oxidation and cause a decrease in quantum yield. Althoughreasonable inks can be made under laboratory conditions, there remainsignificant challenges to replicate this under commercial conditions ona large scale, for example at the mixing stage the QDs need to be stableto air.

The introduction of QDs into a solid state matrix, such as a ‘beadmaterial’, in accordance with the fifth aspect of the present disclosureis of great advantage. QD-beads can be incorporated into a polymermatrix or medium to form a QD-bead ink by dispersing the desired amountof QD-bead material in the desired amount of a suitable polymer. Theresulting composite is mixed thoroughly to provide a homogeneous inkthat can be cured according to the specific curing procedure for thatparticular polymer used and provide a simple and straightforward way offabricating a luminescent QD-bead ink.

QD-bead inks can offer other advantages over free ‘bare’ QD-inks. Byincorporating QDs into stable beads it is possible to protect theotherwise reactive QDs from the potentially damaging surroundingchemical environment. Moreover, by placing a number of QDs into a singlebead, the subsequent QD-bead is more stable, than the bare QDs, tomechanical and thermal processing that the QD-ink often must undergoduring the fabrication of luminescent products. Additional advantages ofQD-containing beads over bare QDs include greater stability to air,moisture and photo-oxidation which might open the possibility ofhandling QD-inks in air and remove the need of expensive handlingprocesses that require an inert atmosphere thus reducing significantlythe manufacturing costs. The size of the beads can be tuned from 50 nmto 0.5 mm in diameter following tailored encapsulation protocols,providing a way to control the ink viscosity. This is very importantbecause the viscosity dictates how an ink flows through a mesh, how itdries, and how well it adheres to a substrate. If the viscosity can becontrolled by the size of the beads, then it is possible to eliminatethe practice of adding significant amounts of thinner to change theviscosity making the process simpler and less expensive.

Because of the nature of the encapsulation process, not only QDaggregation is prevented yielding a uniform layer, but also the QDsurface is not disrupted or drastically modified and the QDs retaintheir original electronic properties so that the specifications of theQD-bead ink can be controlled tightly. QD-beads permit efficient colormixing of the quantum dots in the ink because the mixing can be eitherwithin the QD-containing beads, i.e. each bead contains a number ofdifferent size/color emitting QDs, or a mixture of differently coloredbeads with all the QDs within a specific bead being of the samesize/color, i.e. some beads containing all blue quantum dots, some allgreen quantum dots and some all red quantum dots.

It is possible to encapsulate hydrophobic coated-QDs into beads composedof a hydrophilic polymer to impart novel surface properties (for examplewater solubility). This is of particular importance for makingwater-based QD inks which have many positive qualities and in particularare environmentally friendly. There are many regulations that haveidentified organic solvents typically used as vehicles in printing inksas hazardous. Hazardous waste regulations restrict disposal options forall wastes mixed with solvents from these inks that are usually oforganic in nature (e.g., toluene, ethanol, isopropanol) and highlyflammable. The chemicals that derive from the break-down of these wastesare also toxic and special measures (like for example special filters)have to be employed in the printing industry to trap these chemicals andavoid their release in the environment. Water-based inks provide anattractive alternative to these organic solvents and a mean ofeliminating both pollution and many of the regulatory constraints on theprinting process.

The same concept can be applied to beads composed of oppositely chargedpolymers, e.g., the bead process can be used to modify the QD surface byswitching the surface charge by using an appropriate polymer. QD surfacecharge is an important parameter in nanotoxicity as it has been observedthat a particular charge on the QD surface can trigger the beginning ofcertain destructive molecular pathways via contact activation. Changingthe surface charge via bead encapsulation process can offer a simplemethod to circumvent this problem.

Thus, bead encapsulation can be interpreted as a method for tuning thesurface functionality of the QDs via a simple process that avoids theuse of harsh experimental conditions and therefore limits the potentialdamages that can occur to the QDs and offers more choices in terms ofthe number and types of resins that can be used to disperse and processthe QD-beads.

Under specific experimental conditions the bead coating can beselectively modified or removed during/prior certain stages of the inkpreparation meaning that the ink can be interpreted as a medium todeliver the QDs. Thus QD-beads represent a way to the controlled releaseand delivery of QDs which could be important for example to protect theQDs and separate them from incompatible substances during certain stagesof the printing process or to increase the affinity of the QDs in aspecific ink solvent.

A first preferred embodiment of a QD-bead ink according to the presentdisclosure comprises green light emitting QD-silica beads in apolystyrene/toluene matrix. A polystyrene/toluene mixture is firstformed to which is then added a suitable amount of the QD-beads, in thiscase InP/ZnS core/shell QD-beads. The resulting mixture is thenprocessed (e.g. heating, mixing etc.) to ensure satisfactory dispersionof the QD-bead particles in the polystyrene/toluene mixture to yield atransparent green QD-bead ink.

A second preferred embodiment of a QD-bead ink according to the presentdisclosure comprises red light emitting acrylate beads in an LEDacrylate matrix. A mixture containing an initiator, Irgacure 819,trimethylol propane trimethacrylate (TMPTM) and lauryl methacrylate isinitially formed. InP/ZnS core/shell QD-acrylate beads are thendispersed in the acrylate mixture to yield a red QD-bead ink.

A third preferred embodiment of a QD-bead ink according to the presentdisclosure comprises red light emitting acrylate beads in a flexibleacrylate matrix comprising trimethylol propane trimethacrylate (TMPTM)and polyisobutylene (PIB). In an alternative embodiment, PIB can besubstituted with PIPS. A mixture containing an initiator, Irgacure 819,and TMPTM is formed. A separate mixture of PIB and lauryl methacrylateis also formed. The amount of TMPTM used in this embodiment isrelatively less than the amount used in the second preferred embodimentto ensure that the acrylate matrix is less crosslinked and thereforemore flexible than the acrylate matrix produced in the second preferredembodiment. The two mixtures are then combined to yield a yellowish inkmatrix. InP/ZnS core/shell QD-acrylate beads are then dispersed in theyellowish matrix to yield a red QDbead ink.

QD-bead phosphors can offer several advantages over free ‘bare’ QDphosphors.

By incorporating QDs into stable beads it is possible to protect theotherwise reactive QDs from the potentially damaging surroundingchemical environment. Moreover, by placing a number of QDs into a singlebead, the subsequent QD-bead is more stable, than the bare QDs, tochemical, mechanical, thermal and photo-processing which is requiredwhen incorporating QDs in most commercial applications such as downconvertor, phosphor materials. Additional advantages of QD-containingbeads over bare QDs include greater stability to air, moisture andphoto-oxidation which might open the possibility of making QD phosphorsprocessable in air and remove the need of expensive inert atmospherehandling processes thus reducing significantly the manufacturing costs.The size of the beads can be tuned, e.g. from 50 nm to 0.5 mm indiameter, following tailored encapsulation protocols, providing a way tocontrol the ink viscosity and opening the access to a range ofinexpensive and commercially available deposition techniques.

Because of the nature of the encapsulation process, not only QDaggregation is prevented yielding a uniform layer but also the QDsurface is not disrupted or drastically modified and the QDs retaintheir original electronic properties so that the specifications of theQD-bead phosphor can be controlled tightly. QD-beads permit efficientcolor mixing of the QDs in the phosphor because the mixing can be eitherwithin the QD-containing beads, i.e. each bead contains a number ofdifferent size/color emitting QDs, or a mixture of differently coloredbeads with all the QDs within a specific bead being the same size/color,i.e. some beads containing all green QDs and others containing all redQDs (see FIGS. 5 to 7 below).

It is possible to encapsulate hydrophobic coated-QDs into beads composedof a hydrophilic polymer to impart novel surface properties (for examplewater solubility). This is of particular importance for makingwater-based QD inks. The same concept can be applied to beads composedof oppositely charged polymers. This can be interpreted as a method fortuning the surface functionality of the QDs via a simple process thatavoids the use of harsh conditions and therefore limits the potentialdamages that can occur to the QDs and can offer more choices in terms ofthe number and type of available resins that can be used to disperse andprocess the QD-beads for fabricating phosphor devices.

Bead encapsulation can help reducing the formation of strains that oftenaffect the phosphor sheets made by the conventional encapsulationmethods and have a detrimental effect on the optical properties of thesheet. In addition, no further film encapsulation is required becausethe QDs in the film are already encapsulated by the surrounding beadpotentially halving the costs of current manufacturing processes thatrequire a final film encapsulation.

Under specific experimental conditions the bead coating can beselectively modified or removed during/prior certain stages of thephosphor sheet preparation meaning that the QD-bead ink can be used as amedium to deliver the QDs. Thus QD-beads represent a way to thecontrolled release and delivery of QDs which could be important forexample for protecting the QDs and separating them from incompatiblesubstances during certain stages of the manufacturing process or forexample for dispersing water-insoluble QDs in an aqueous medium morereadily.

An important achievement described in the present disclosure is theencapsulation of QDs in an encapsulation medium that confers stabilityto the QDs but without altering their optical properties and theirprocessability. Embedding colloidal QDs in a host matrix has the majoradvantage of protecting the QDs from their surrounding chemicalenvironment, air, moisture and oxygen and increasing theirphoto-stability. However, one of the challenges is to find a transparenthost matrix that can act as a conductive layer and be non-emitting e.g.,does not interfere with the light emitted by the primary source (e.g.LED) and the light emitted by the QDs. The polymer matrix needs to bestable under intense illumination and high energy (i.e. UV source) andfor some applications needs to have also some stability at elevatedtemperatures.

Aspects of the present disclosure relate to QD-bead phosphor sheets madefrom QD containing bead architectures and methods of producing QD-beadphosphor sheets.

A first preferred embodiment of a method for fabricating a QD-beadphosphor sheet in accordance with an aspect of the present disclosureemploys green silica beads in a polystyrene/toluene matrix. Two spacersare fixed to a polyethylene terephthalate (PET) sheet with a constantgap (e.g. 15 mm) defined between them. A predetermined volume of aQD-bead ink, such as the ink described above in the first preferredembodiment of a QDbead ink, is then drop cast on to the region of thePET sheet in between the spacers. The ink is then distributed uniformlybetween the spacers and then heated to remove the solvent. The resultingfilm exhibited noticeable fluorescence under bright ambient lightconditions.

A second preferred embodiment of a method for fabricating a QD-beadphosphor sheet in accordance with an aspect of the present disclosureemploys red acrylate beads in an LED acrylate matrix. A predeterminedvolume of a QD-bead ink (e.g. the ink in accordance with the secondpreferred embodiment of an ink described above) is drop cast on to aglass mould and then cured to yield a QD-bead polymer film.

A third preferred embodiment of a method for fabricating a QD-beadphosphor sheet in accordance with an aspect of the present disclosureemploys red acrylate beads in a flexible acrylate matrix. Apredetermined volume of a QD-bead ink (e.g. the ink in accordance withthe third preferred embodiment of an ink described above) is drop caston to a glass mould and then cured to yield a QD-bead polymer film.

All three of the above preferred embodiments successfully producedQD-bead phosphor sheets exhibiting good optical performance.

The current disclosure describes the preparation of a QD phosphor sheetmade of QDs embedded into an optically clear and chemically stablemedium termed “beads”—the term beads as used herein can mean anythree-dimensional shape, constituency or size of material—using avariety of techniques. Preparation of beads can be achieved by severalprocesses including by incorporating the QDs directly into the polymermatrices of resin beads or by immobilizing the QDs in polymer beadsthrough physical entrapment.

In the semiconductor nanoparticle materials employed in the variousaspects of the present disclosure the core material may comprise any oneor more of the following types of material.

II-IV compounds including a first element from group 12 (II) of theperiodic table and a second element from group 16 (VI) of the periodictable, as well as ternary and quaternary materials including, but notrestricted to, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS,CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS,CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS,CdZnSeTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe.

II-V compounds incorporating a first element from group 12 of theperiodic table and a second element from group 15 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: Zn₃P₂, Zn₃As₂,Cd₃P₂, Cd₃As₂, Cd₃N₂, Zn₃N₂.

III-V compounds including a first element from group 13 (III) of theperiodic table and a second element from group 15 (V) of the periodictable, as well as ternary and quaternary materials. Examples ofnanoparticle core materials include but are not restricted to: BP, AlP,AlAs, AlSb; GaN, GaP, GaAs, GaSb; InN, InP, InAs, InSb, AlN, BN, GaNP,GaNAs, InNP, InNAs, GAInPAs, GaAlPAs, GaAlPSb, GaInNSb, InAlNSb,InAlPAs, InAlPSb.

III-VI compounds including a first element from group 13 of the periodictable and a second element from group 16 of the periodic table and alsoincluding ternary and quaternary materials. Nanoparticle materialincludes but is not restricted to: Al₂S₃, Al₂Se₃, Al₂Te₃, Ga₂S₃, Ga₂Se₃,In₂S₃, In₂Se₃, Ga₂Te₃, In₂Te₃.

IV compounds including elements from group 14 (IV) Si, Ge, SiC and SiGe.

IV-VI compounds including a first element from group 14 (IV) of theperiodic table and a second element from group 16 (VI) of the periodictable, as well as ternary and quaternary materials including, but notrestricted to, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe,PbSTe, SnPbSe, SnPbTe, SnPbSeTe, SnPbSTe.

The material of any buffer layer or shell layer(s) grown on thenanoparticle core may include any one or more of the followingmaterials.

IIA-VIB (2-16) material, incorporating a first element from group 2 ofthe periodic table and a second element from group 16 of the periodictable and also including ternary and quaternary materials and dopedmaterials. Nanoparticle material includes but is not restricted to: MgS,MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe.

IIB-VIB (12-16) material incorporating a first element from group 12 ofthe periodic table and a second element from group 16 of the periodictable and also including ternary and quaternary materials and dopedmaterials. Nanoparticle material includes but is not restricted to: ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe.

II-V material incorporating a first element from group 12 of theperiodic table and a second element from group 15 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: Zn₃P₂, Zn₃As₂,Cd₃P₂, Cd₃As₂, Cd₃N₂, Zn₃N₂.

III-V material incorporating a first element from group 13 of theperiodic table and a second element from group 15 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: BP, AlP, AlAs,AlSb; GaN, GaP, GaAs, GaSb; InN, InP, InAs, InSb, AlN, BN.

III-IV material incorporating a first element from group 13 of theperiodic table and a second element from group 14 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: B₄C, Al₄C₃,Ga₄C.

III-VI material incorporating a first element from group 13 of theperiodic table and a second element from group 16 of the periodic tableand also including ternary and quaternary materials. Nanoparticlematerial includes but is not restricted to: Al₂S₃, Al₂Se₃, Al₂Te₃,Ga₂S₃, Ga₂Se₃, In₂S₃, In₂Se₃, Ga₂Te₃, In₂Te₃.

IV-VI material incorporating a first element from group 14 of theperiodic table and a second element from group 16 of the periodic tableand also including ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: PbS, PbSe,PbTe, Sb₂Te₃, SnS, SnSe, SnTe.

Nanoparticle material incorporating a first element from any group inthe transition metal of the periodic table, and a second element fromany group of the d-block elements of the periodic table and alsoincluding ternary and quaternary materials and doped materials.Nanoparticle material includes but is not restricted to: NiS, CrS,CuInS₂, CuInSe₂, CuGaS₂, CuGaSe₂.

QDs of III-V semiconductors have reduced toxicity compared to IIB-VIsemiconductor compounds offering a potential substitute to the widelyused cadmium-based QDs. Nevertheless, the study and application of III-Vsemiconductor QDs are limited by the difficulty in their synthesis.Although InP is the most extensively studied semiconductor materialwithin the III-V group, the synthesis of the InP QDs and in general ofIII-V semiconductors by traditional chemical methods present in theprior art does not yield QDs of the same optical and physical propertiesas those of most IIB-VI semiconductor nanocrystals, including CdSe andCdS. The QDs made according these traditional chemical methods arecharacterized by poor electronic properties including relatively lowPLQY. These limitations hinder significantly the utilization of heavymetal-free semiconductor QDs in emitting devices. Another concern forthe electronic industry is the need of a supply of QDs in multi-gramquantities for the mass production of commercial products and theconventional methods can deliver usually only micro-gram quantities ofthese materials.

“Capping agent”—Outermost Particle Layer

The coordination around the atoms on the surface of any core, core-shellor core-multi shell, doped or graded nanoparticle is incomplete and thenon-fully coordinated atoms have dangling bonds which make them highlyreactive and can lead to particle agglomeration. This problem isovercome by passivating (capping) the “bare” surface atoms withprotecting organic groups.

The outermost layer (capping agent) of organic material or sheathmaterial helps to inhibit particle-particle aggregation, furtherprotects the nanoparticle from their surrounding electronic and chemicalenvironments and also provides a mean of chemical linkage to otherinorganic, organic or biological material. In many cases, the cappingagent is the solvent that the nanoparticle preparation is undertaken in,and consists of a Lewis base compound, or a Lewis base compound dilutedin an inert solvent such as a hydrocarbon. There is a lone pair ofelectrons on the Lewis base capping agent that are capable of a donortype coordination to the surface of the nanoparticle and include mono-or multi-dentate ligands such as phosphines (trioctylphosphine,triphenylphosphine, t-butylphosphine etc.), phosphine oxides(trioctylphosphine oxide, triphenylphosphine oxide etc.), alkylphosphonic acids, alkylamines (octadecylamine, hexadecylamine,octylamine etc.), aryl-amines, pyridines, long chain fatty acids(myristic acid, oleic acid, undecylenic acid etc.) and thiophenes butis, as one skilled in the art will know, not restricted to thesematerials.

Surface-Modified QDs

The outermost layer (capping agent) of a QD can also consist of acoordinated ligand with additional functional groups that can be used aschemical linkage to other inorganic, organic or biological material,whereby the functional group is pointing away from the QD surface and isavailable to bond/react/interact with other available molecules, such asamines, alcohols, carboxylic acids, esters, acid chloride, anhydrides,ethers, alkyl halides, amides, alkenes, alkanes, alkynes, allenes, aminoacids, azides, groups etc. but is, as one skilled in the art will know,not limited to these functionalised molecules. The outermost layer(capping agent) of a QD can also consist of a coordinated ligand with afunctional group that is polymerisable and can be used to form a polymerlayer around the particle.

The outermost layer (capping agent) can also consist of organic unitsthat are directly bonded to the outermost inorganic layer such as via anS—S bond between the inorganic surface (ZnS) and a thiol cappingmolecule. These can also possess additional functional group(s), notbonded to the surface of the particle, which can be used to form apolymer around the particle, or for furtherreaction/interaction/chemical linkage.

QD Beads (QD-Beads)

The photo-stability of QDs employed in the various aspects of thepresent disclosure is increased by incorporating the QDs into opticallytransparent beads. Considering the initial step of incorporating QDsinto beads, a first option is to incorporate the QDs directly into thepolymer matrices of resin beads. A second option is to immobilise theQDs in polymer beads through physical entrapment. It is possible usingthese methods to make a population of beads that contain just a singletype of QD (e.g. one color) by incorporating a single type of QD intothe beads. Alternatively, it is possible to construct beads that contain2 or more types of QD (e.g. two or more colors) by incorporating amixture of two or more types of QD (e.g. material and/or size) into thebeads. Such mixed beads can then be combined in any suitable ratio toemit any desirable color of secondary light following excitation by theprimary light emitted by the primary light source (e.g. LED). This isexemplified in FIGS. 5 to 7 which schematically show QD-bead lightemitting devices including respectively: a) beads includingmulti-colored QDs such that each bead emits white secondary light; b)multiple beads, each bead containing a single color of QD such that eachbead emits light of a single color but the combination of differentlycolored beads produce white secondary light; and c) beads containing asingle color of QD such that a mixture of the beads emits a single colorof secondary light, e.g. red.

Incorporation of QDs During Bead Formation

With regard to the first option, by way of example,hexadecylamine-capped CdSe based semiconductor nanoparticles can betreated with at least one, more preferably two or more polymerisableligands (optionally one ligand in excess) resulting in the displacementof at least some of the hexadecylamine capping layer with thepolymerisable ligand(s). The displacement of the capping layer with thepolymerisable ligand(s) can be accomplished by selecting a polymerisableligand or ligands with structures similar to that of trioctylphosphineoxide (TOPO), which is a ligand with a known and very high affinity forCdSe-based nanoparticles. It will be appreciated that this basicmethodology may be applied to other nanoparticle/ligand pairs to achievea similar effect. That is, for any particular type of nanoparticle(material and/or size), it is possible to select one or more appropriatepolymerisable surface binding ligands by choosing polymerisable ligandscomprising a structural motif which is analogous in some way (e.g. has asimilar physical and/or chemical structure) to the structure of a knownsurface binding ligand. Once the nanoparticles have beensurface-modified in this way, they can then be added to a monomercomponent of a number of microscale polymerisation reactions to form avariety of QD-containing resins and beads. Another option is thepolymerisation of one or more polymerisable monomers from which theoptically transparent medium is to be formed in the presence of at leasta portion of the semiconductor nanoparticles to be incorporated into theoptically transparent medium. The resulting materials incorporate theQDs covalently and appear highly colored even after prolonged periods ofSoxhlet extraction.

Examples of polymerisation methods that may be used to constructQD-containing beads include suspension, dispersion, emulsion, living,anionic, cationic, RAFT, ATRP, bulk, ring closing metathesis and ringopening metathesis but not exclusive to. Initiation of thepolymerisation reaction may be caused by any suitable method whichcauses the monomers to react with one another, such as by the use offree radicals, light, ultrasound, cations, anions, or heat. A preferredmethod is suspension polymerisation involving thermal curing of one ormore polymerisable monomers from which the optically transparent mediumis to be formed. Said polymerisable monomers preferably comprise methyl(meth)acrylate, ethylene glycol dimethacrylate and vinyl acetate. Thiscombination of monomers has been shown to exhibit excellentcompatibility with existing commercially available LED encapsulants andhas been used to fabricate a light emitting device exhibitingsignificantly improved performance compared to a device prepared usingessentially prior art methodology. Other preferred polymerisablemonomers are epoxy or polyepoxide monomers, which may be polymerisedusing any appropriate mechanism, such as curing with ultravioletirradiation.

QD-containing microbeads can be produced by dispersing a knownpopulation of QDs within a polymer matrix, curing the polymer and thengrinding the resulting cured material. This is particularly suitable foruse with polymers that become relatively hard and brittle after curing,such as many common epoxy or polyepoxide polymers (e.g. Optocast™ 3553from Electronic Materials, Inc., USA).

QD-containing beads may be generated simply by adding QDs to the mixtureof reagents used to construct the beads. In some instances QDs (nascentQDs) will be used as isolated from the reaction employed to synthesisethem and are thus generally coated with an inert outer organic ligandlayer. In an alternative procedure a ligand exchange process may becarried out prior to the bead forming reaction. Here one or morechemically reactive ligands (for example this might be a ligand for theQDs which also contains a polymerisable moiety) are added in excess to asolution of nascent QDs coated in an inert outer organic layer. After anappropriate incubation time the QDs are isolated, for example byprecipitation and subsequent centrifugation, washed and thenincorporated into the mixture of reagents used in the bead formingreaction/process.

Both QD incorporation strategies will result in statistically randomincorporation of the QDs into the beads and thus the polymerisationreaction will result in beads containing statistically similar amountsof the QDs. It will be obvious to one skilled in the art that bead sizecan be controlled by the choice of polymerisation reaction used toconstruct the beads and additionally once a polymerisation method hasbeen selected bead size can also be controlled by selecting appropriatereaction conditions, e.g. in a suspension polymerisation reaction bystirring the reaction mixture more quickly to generate smaller beads.Moreover the shape of the beads can be readily controlled by choice ofprocedure in conjunction with whether or not the reaction is carried outin a mould. The composition of the beads can be altered by changing thecomposition of the monomer mixture from which the beads are constructed.Similarly the beads can also be cross-linked with varying amounts of oneor more cross-linking agents (e.g. divinyl benzene). If beads areconstructed with a high degree of cross-linking, e.g. greater than 5 mol% cross-linker, it may be desirable to incorporate a porogen (e.g.toluene or cyclohexane) during the reaction used to construct the beads.The use of a porogen in such a way leaves permanent pores within thematrix constituting each bead. These pores may be sufficiently large toallow the ingress of QDs into the bead.

QDs can also be incorporated in beads using reverse emulsion basedtechniques. The QDs may be mixed with precursor(s) to the opticallytransparent coating material and then introduced into a stable reverseemulsion containing, for example, an organic solvent and a suitablesalt. Following agitation the precursors form microbeads encompassingthe QDs, which can then be collected using any appropriate method, suchas centrifugation. If desired, one or more additional surface layers orshells of the same or a different optically transparent material can beadded prior to isolation of the QD-containing beads by addition offurther quantities of the requisite shell layer precursor material(s).

Incorporation of QDs into Prefabricated Beads

In respect of the second option for incorporating QDs into beads, theQDs can be immobilised in polymer beads through physical entrapment. Forexample, a solution of QDs in a suitable solvent (e.g. an organicsolvent) can be incubated with a sample of polymer beads. Removal of thesolvent using any appropriate method results in the QDs becomingimmobilised within the matrix of the polymer beads. The QDs remainimmobilised in the beads unless the sample is resuspended in a solvent(e.g. organic solvent) in which the QDs are freely soluble. Optionally,at this stage the outside of the beads can be sealed. Another option isto physically attach at least a portion of the semiconductornanoparticles to prefabricated polymeric beads. Said attachment may beachieved by immobilisation of the portion of the semiconductornanoparticles within the polymer matrix of the prefabricated polymericbeads or by chemical, covalent, ionic, or physical connection betweenthe portion of semiconductor nanoparticles and the prefabricatedpolymeric beads. Examples of prefabricated polymeric beads comprisepolystyrene, polydivinyl benzene and a polythiol.

QDs can be irreversibly incorporated into prefabricated beads in anumber of ways, e.g. chemical, covalent, ionic, physical (e.g. byentrapment) or any other form of interaction. If prefabricated beads areto be used for the incorporation of QDs, the solvent accessible surfacesof the bead may be chemically inert (e.g. polystyrene) or alternativelythey may be chemically reactive/functionalised (e.g. Merrifield'sResin). The chemical functionality may be introduced during theconstruction of the bead, for example by the incorporation of achemically functionalised monomer, or alternatively chemicalfunctionality may be introduced in a post bead construction treatmentfor example by conducting a chloromethylation reaction. Additionallychemical functionality may be introduced by a post bead constructionpolymeric graft or other similar process whereby chemically reactivepolymer(s) are attached to the outer layers/accessible surfaces of thebead. More than one such post construction derivatisation process may becarried out to introduce chemical functionality onto/into the bead.

As with QD incorporation into beads during the bead forming reaction,i.e. the first option described above, the pre-fabricated beads can beof any shape, size and composition and may have any degree ofcross-linker and may contain permanent pores if constructed in thepresence of a porogen. QDs may be imbibed into the beads by incubating asolution of QDs in an organic solvent and adding this solvent to thebeads. The solvent must be capable of wetting the beads and in the caseof lightly crosslinked beads, preferably 0-10% crosslinked and mostpreferably 0-2% crosslinked the solvent should cause the polymer matrixto swell in addition to solvating the QDs. Once the QD-containingsolvent has been incubated with the beads, it is removed (for example byheating the mixture and causing the solvent to evaporate) and the QDsbecome embedded in the polymer matrix constituting the bead oralternatively by the addition of a second solvent in which the QDs arenot readily soluble but which mixes with the first solvent causing theQDs to precipitate within the polymer matrix constituting the beads.Immobilization may be reversible if the bead is not chemically reactiveor else if the bead is chemically reactive the QDs may be heldpermanently within the polymer matrix, by chemical, covalent, ionic, orany other form of interaction.

Incorporation of QDs into Sol-Gels to Produce Glass

Optically transparent media which are sol-gels and glasses that areintended to incorporate QDs may be formed in an analogous fashion to themethod used to incorporate QDs into beads during the bead formingprocess as described above. For example, a single type of QD (e.g. onecolor) may be added to the reaction mixture used to produce the solgelor glass. Alternatively, two or more types of QD (e.g. two or morecolors) may be added to the reaction mixture used to produce the sol-gelor glass. The sol-gels and glasses produced by these procedures may haveany shape, morphology or 3-dimensional structure. For example, theparticles may be spherical, disc-like, rod-like, ovoid, cubic,rectangular or any of many other possible configurations.

Stability-Enhancing Additives

By incorporating QDs into beads in the presence of materials that act asstability enhancing additives, and optionally providing the beads with aprotective surface coating, migration of deleterious species, such asmoisture, oxygen and/or free radicals, is eliminated or at leastreduced, with the result of enhancing the physical, chemical and/orphoto-stability of the semiconductor nanoparticles.

An additive may be combined with “naked” semiconductor nanoparticles andprecursors at the initial stages of the production process of the beads.Alternatively, or additionally, an additive may be added after thesemiconductor nanoparticles have been entrapped within the beads.

The additives which may be added singly or in any desirable combinationduring the bead formation process can be grouped according to theirintended function as follows:

Mechanical sealing: Fumed silica (e.g. Cab-O-Sil® CABOT CORPORATION, TWOSEAPORT LANE, SUITE 1300, BOSTON MASS. 02210-2019), ZnO, TiO₂, ZrO, Mgstearate, Zn Stearate, all used as a filler to provide mechanicalsealing and/or reduce porosity;

Capping agents: Tetradecyl phosphonic acid (TDPA), oleic acid, stearicacid, polyunsaturated fatty acids, sorbic acid. Zn methacrylate, Mgstearate, Zn Stearate, isopropyl myristate. Some of these have multiplefunctionality and can act as capping agents, free radical scavengersand/or reducing agents;

Reducing agents: Ascorbic acid palmitate, alpha tocopherol (vitamin E),octane thiol, butylated hydroxyanisole (BHA), butylated hydroxytoluene(BHT), gallate esters (propyl, lauryl, octyl and the like), and ametabisulfite (e.g. the sodium or potassium salt);

Free radical scavengers: benzophenones; and

Hydride reactive agents: 1,4-butandiol, 2-hydroxyethyl methacrylate,allyl methacrylate, 1,6 heptadiene-4-ol, 1,7 octadiene, and 1,4butadiene.

The selection of the additive or additives for a particular applicationwill depend upon the nature of the semiconductor nanoparticle material(e.g. how sensitive the nanoparticle material is to physical, chemicaland/or photo-induced degradation), the nature of the primary matrixmaterial (e.g. how porous it is to potentially deleterious species, suchas free-radicals, oxygen, moisture etc.), the intended function of thefinal material or device which will contain the primary particles (e.g.the operating conditions of the material or device), and the processconditions required to fabricate said final material or device. Withthis in mind, one or more appropriate additives can be selected from theabove five lists to suit any desirable semiconductor nanoparticleapplication.

Bead Surface Coating Materials

Once the QDs are incorporated into the beads, the formed QD-beads can befurther coated with a suitable material to provide each bead with aprotective barrier to prevent the passage or diffusion of potentiallydeleterious species, e.g. oxygen, moisture or free radicals, from theexternal environment through the bead material to the semiconductornanoparticles. As a result, the semiconductor nanoparticles are lesssensitive to their surrounding environment and the various processingconditions typically required to utilise the nanoparticles inapplications such as the fabrication of phosphor sheets and devicesincorporating such sheets.

The coating is preferably a barrier to the passage of oxygen or any typeof oxidising agent through the bead material. The coating may be abarrier to the passage of free radical species, and/or is preferably amoisture barrier so that moisture in the environment surrounding thebeads cannot contact the semiconductor nanoparticles incorporated withinthe beads.

The coating may provide a layer of coating material on a surface of thebead of any desirable thickness provided it affords the required levelof protection. The surface layer coating may be around 1 to 10 nm thick,up to around 400 to 500 nm thick, or more. Preferred layer thicknessesare in the range 1 nm to 200 nm, more preferably around 5 to 100 nm.

The coating can comprise an inorganic material, such as a dielectric(insulator), a metal oxide, a metal nitride or a silica-based material(e.g. a glass).

The metal oxide may be a single metal oxide (i.e. oxide ions combinedwith a single type of metal ion, e.g. Al₂O₃), or may be a mixed metaloxide (i.e. oxide ions combined with two or more types of metal ion,e.g. SrTiO₃). The metal ion(s) of the (mixed) metal oxide may beselected from any suitable group of the periodic table, such as group 2,13, 14 or 15, or may be a transition metal, d-block metal, or lanthanidemetal.

Preferred metal oxides are selected from the group consisting of Al₂O₃,B₂O₃, Co₂O₃, Cr₂O₃ , CuO, Fe₂O₃, Ga₂O₃, HfO₂, In₂O₃ , MgO, Nb₂O₅ , NiO,SiO₂, SnO₂, Ta₂O₅, TiO₂, ZrO₂, Sc₂O₃, Y₂O₃, GeO₂, La₂O₃, CeO₂, PrO_(x)(x=appropriate integer), Nd₂O₃, Sm₂O₃, EuO_(y) (y=appropriate integer),Gd₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, SrTiO₃, BaTiO₃, PbTiO₃,PbZrO₃, Bi_(m)Ti_(n)O (m=appropriate integer; n=appropriate integer),Bi_(a)Si_(b)O (a=appropriate integer; b=appropriate integer), SrTa₂O₆,SrBi₂Ta₂O₉, YScO₃, LaAlO₃, NdAlO₃, GdScO₃, LaScO₃, LaLuO₃, Er₃Ga₅O₁₃.

Preferred metal nitrides may be selected from the group consisting ofBN, AlN, GaN, InN, Zr₃N₄, Cu₂N, Hf₃N₄, SiN_(c) (c=appropriate integer),TiN, Ta₃N₅, Ti—Si—N, Ti—Al—N, TaN, NbN, MoN, WN_(d) (d=appropriateinteger), WN_(e)C_(f) (e=appropriate integer; f=appropriate integer).

The inorganic coating may comprise silica in any appropriate crystallineform.

The coating may incorporate an inorganic material in combination with anorganic or polymeric material, e.g., an inorganic/polymer hybrid, suchas a silica-acrylate hybrid material.

The coating can comprise a polymeric material which may be a saturatedor unsaturated hydrocarbon polymer, or may incorporate one or moreheteroatoms (e.g. O, S, N, halo) or heteroatom-containing functionalgroups (e.g. carbonyl, cyano, ether, epoxide, amide and the like).

Examples of preferred polymeric coating materials include acrylatepolymers (e.g. polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacryaltes, polyethyleneglycoldimethacrylate, polyvinylacetate etc), epoxides (e.g., EPOTEK 301 A+ BThermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), polyetheretherketone (PEEK),polyvinylidene fluoride (PVDF), polydivinyl benzene, polyethylene,polypropylene, polyethylene terephthalate (PET), polyisobutylene (butylrubber), polyisoprene, and cellulose derivatives (methyl cellulose,ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Aspects of the present disclosure relate to a phosphor layer or sheetmade of QDs dispersed in a polymeric matrix in the form of beads. TheseQD-containing beads have a number of advantages. The beads can preventthe aggregation of the QDs and lead to emitting layers with improvedperformance. The beads can be made by a simple process that avoids theuse of harsh conditions and therefore limits the potential damages thatcan occur to the QDs during their incorporation. The result is that theQDs embedded in the beads retain their original electronic properties,with the additional benefits of increased protection compared to thebare dots from the surrounding chemical environment and photo-oxidation.This results in a greater tolerance to the processing conditionsnecessary for incorporation into solid state structures which cantranslate into a reduction in the overall manufacturing costs. Theability to incorporate the QDs into variety of polymers provides theability to improve the dispersibility and processability of the QDmaterials in a wide range of resins (both hydrophobic and hydrophilic),opening up new opportunities for the fabrication of phosphor layers inapplications such as lighting and display technology.

EXPERIMENTAL SECTION

Set out below is a description of methods for producing QDs (includingheavy metal-free QDs), their incorporation into beads, the formulationof QD-bead containing inks in accordance with aspects of the presentdisclosure and methods for the fabrication of QD phosphor sheets, layersor films from QD-beads in accordance with further aspects of the presentdisclosure.

Reference Example 1: Preparation of CdSe/ZnS Core/Shell QDs Preparationof CdSe Core QDs

Hexadecylamine (HDA, 500 g) was placed in a three-neck round bottomedflask and degassed by heating to 120° C. under a dynamic vacuum for >1hour. The solution was then cooled to 60° C. and[HNEt₃]₄[Cd₁₀Se₄(SPh)₁₆] (0.718 g, 0.20 mmols) was added through aside-port under a strong flow of nitrogen. TOPSe and Me₂Cd.TOP (4 mmolseach) were added dropwise into the reaction vessel, the temperature wasincreased to 110° C. and the reaction was allowed to stir for 2 hoursafter which time the solution was of a deep yellow color. Furtherdropwise additions of equimolar amounts of TOPSe and Me₂Cd.TOP werecarried out while the temperature was progressively increased at a rateof 0.2° C./min. In total 42 mmols of TOPSe and 42 mmols of Me₂Cd.TOPwere used. The reaction was stopped when the PL emission maximum hadreached ˜600 nm, by cooling to 60° C. followed by addition of 300 mL ofdry ethanol or acetone. This produced a precipitation of deep redparticles, which were further isolated by filtration. The resulting CdSeQDs were recrystallized by re-dissolving them in toluene, filtering themthrough Celite followed by reprecipitation with warm ethanol to removeany excess of HDA and any un-reacted subproducts. This produced 10.10 gof HDA-capped CdSe QDs with luminescence emission maximum=585 nm andFWHM (full width at half maximum)=35 nm.

Growth of a ZnS Shell on CdSe Core QDs

HDA (800 g) was placed in a three-neck round-bottom flask, dried anddegassed by heating to 120° C. under a dynamic vacuum for >1 hour. Afterthe solution was then cooled to 60° C. 9.23 g of CdSe QDs as preparedabove were added. The HDA was then heated to 220° C. before adding atotal of 20 mL of a 0.5 M solution of Me₂Zn.TOP and 20 mL of a 0.5 Msolution of sulfur in octylamine by dropwise injections. Three alternateinjections of 3.5 mL, 5.5 mL and 11.0 mL of each solution were made,whereby initially 3.5 mL of sulfur was added dropwise until theintensity of the PL maximum was near zero. Then 3.5 mL of Me₂Zn.TOP wasadded dropwise until the intensity of the PL maximum had reached amaximum. This cycle was repeated with the PL maximum reaching a higherintensity after each cycle. After a PL maximum intensity was reached onthe last cycle, additional shelling reagents were added until the PLintensity was between 5-10% below its maximum value, and the reactionwas allowed to anneal at 150° C. for 1 hour. The reaction mixture wasthen allowed to cool to 60° C. whereupon 300 mL of dry “warm” ethanolwas added which resulted in the precipitation of particles. Theresulting CdSe/ZnS QDs were re-dissolved in toluene and filtered throughCelite followed by re-precipitation from warm ethanol to remove anyexcess HDA. This produced 12.08 g of HDA-capped CdSe/ZnS core/shell QDswith luminescence emission maximum=590 nm and FWHM=36 nm (see FIG. 8).The photoluminescence quantum yield efficiencies (PLQY) of thecore/shell materials at this stage range from 50 to 90%.

Reference Example 2: Preparation of InP/ZnS Core/Shell QDs Preparationof InP Core QDs (400-800 nm Emission)

Di-butyl ester (100 mL) and myristic acid (10.1 g) were placed in athree-neck flask and degassed at 70° C. under vacuum for one hour. Afterthis period, nitrogen was introduced and the temperature increased to90° C. ZnS molecular cluster [Et₃NH]₄[Zn₁₀S₄(SPh)₁₆] (4.7 g) was addedand the mixture allowed to stir for 45 minutes. The temperature was thenincreased to 100° C. followed by the dropwise addition of Indiummyristate, In(MA)₃ (1 M in ester, 15 mL) followed by (TMS)₃P (1 M inester, 15 mL). The reaction mixture was allowed to stir while increasingthe temperature to 140° C. At 140° C., further dropwise additions ofIn(MA)₃ (1 M, 35 mL) (left to stir for 5 min) and (TMS)₃P (1 M, 35 mL)were made. The temperature was then slowly increased to 180° C. andfurther dropwise additions of In(MA)₃ (1 M, 55 mL) followed by (TMS)₃P(1 M, 40 mL) were made. By addition of the precursor in the mannerdescribed above InP QDs could be grown with the emission peak positiongradually increasing from 500 nm up to 750 nm, whereby the reaction canbe stopped when the desired emission peak position has been reached andleft to stir at this temperature for half an hour. After this period,the temperature was decreased to 160° C. and the reaction mixture wasleft to anneal for up to 4 days (at a temperature of 20 to 40° C. belowthat of the reaction). A UV lamp was also used at this stage to aid theannealing process.

The nanoparticles were isolated by the addition of dried degassedmethanol (approx. 200 mL) via cannula techniques. The precipitate wasallowed to settle and then methanol was removed via cannula with the aidof a filter stick. Dried degassed chloroform (approx. 10 mL) was addedto wash the solid. The solid was left to dry under vacuum for 1 day.This produced 5.60 g of InP core nanoparticles with luminescenceemission maximum=630 nm and FWHM=70 nm.

Post-Operative Treatments of InP Core QDs

The PLQY of the InP QDs prepared by the method above was increased bytreatment with dilute hydrofluoric acid (HF) acid. The QDs weredissolved in anhydrous degassed chloroform (˜270 mL) and a 50 mL portionwas withdrawn and placed in a polypropylene flask. The HF solution wasprepared by injecting with a disposable polypropylene syringe 3 mL of a60% w/w HF solution in water in 17 mL of pre-degassed THF.

Reference Example 3: Incorporation of QDs into Suspension PolymericBeads

1% wt/vol polyvinyl acetate (PVA) aqueous solution was prepared bystirring for 12 hours followed by extensive degassing by bubblingnitrogen through the solution for a minimum of 1 hour. The monomers,methyl methacrylate and ethylene glycol dimethacrylate, were alsodegassed by nitrogen bubbling and used with no further purification. Theinitiator AIBN (0.012 g) was placed into the reaction vessel and putunder three vacuum/nitrogen cycles to ensure no oxygen was present.

CdSe/ZnS core/shell QDs as prepared above were added to the reactionvessel as a solution in toluene and the solvent was removed underreduced pressure. Degassed methyl methacrylate (0.98 mL) was then addedfollowed by degassed ethylene glycol dimethacrylate (0.15 mL). Themixture was then stirred at 800 rpm for 15 minutes to ensure completedispersion of the QDs within the monomer mixture. The solution of 1% PVA(10 mL) was then added and the reaction stirred for 10 minutes to ensurethe formation of the suspension. The temperature was then raised to 72°C. and the reaction allowed to proceed for 12 hours.

The reaction mixture was then cooled to room temperature and the beadedproduct washed with water until the washings ran clear followed bymethanol (100 mL), methanol/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/dichloromethane (1:1, 100 mL),dichloromethane (100 mL), dichloromethane/tetrahydrofuran (1:1, 100 mL),tetrahydrofuran (100 mL), tetrahydrofuran/methanol (1:1, 100 mL),methanol (100 mL). The QD-containing beads (QD-beads) were then driedunder vacuum and stored under nitrogen.

Reference Example 4: Adsorption of QDs into Prefabricated Beads

Polystyrene microspheres with 1% divinyl benzene (DVB) and 1% thiolcomonomer were suspended in toluene (1 mL) by shaking and sonication.The microspheres were centrifuged (6000 rpm, approx. 1 min) and thesupernatant decanted. This was repeated for a second wash with tolueneand the pellets then resuspended in toluene (1 mL).

InP/ZnS QDs as prepared above were dissolved (an excess, usually 5 mg ofQDs for 50 mg of microspheres) in chloroform (0.5 mL) and filtered toremove any insoluble material. The QD-chloroform solution was added tothe microspheres in toluene and shaken on a shaker plate at roomtemperature for 16 hours to mix thoroughly.

The QD-microspheres were centrifuged to a pellet and the supernatantdecanted off, which contained any excess QDs present. The pellet waswashed (as above) twice with toluene (2 mL), resuspended in toluene (2mL), and then transferred directly into a glass vial. The glass vial wasplaced inside a centrifuge tube, centrifuged and the excess of toluenewas decanted.

Reference Example 5: Incorporation of QDs into Silica Beads

InP/ZnS core/shell QDs (70 mg) as prepared above were mixed with 0.1 mLof (3(trimethoxysilyl)propyl methacrylate (TMOPMA), followed by 0.5 mLof tetraethoxyorthosilicate (TEOS) to form a clear solution, which waskept for incubation under N₂ overnight. The mixture was then injectedinto 10 mL of a reverse microemulsion (cyclohexane/CO-520, 18 mL/1.35 g)in 50 mL flask, under stirring at 600 rpm. The mixture was stirred for15 mins and then 0.1 mL of 4% NH₄OH was injected to start the beadforming reaction. The reaction was stopped the next day and the reactionsolution was centrifuged to collect the solid phase. The obtainedparticles were washed twice with 20 mL cyclohexane and then dried undervacuum.

EXAMPLES Example 1—Formulation of a Semiconductor QD-Bead Ink (GreenSilica Beads in Polystyrene/Toluene Matrix)

Inside a nitrogen-filled glovebox toluene (25 g) was injected into aglass vial with a magnetic stirrer and the vial was sealed. The vial wasplaced on a hot plate and toluene was heated at 60° C. with stirring at250 rpm for 5 min before a predetermined amount of the polystyrene resin(8.3 g) was added. Once all polystyrene resin was dissolved the speed ofthe stirrer was reduced down to 150 rpm, the temperature was decreasedto 40° C. and the resulting mixture was left to stir for 12 hrs. Afterthis period, inside the nitrogen-filled glove box, 2 g of the lattersolution were transferred to a smaller glass vial. A magnetic stirrerwas introduced in the vial which was then sealed and placed on a hotplate pre-heated to 60° C. The latter solution was stirred at 250 rpmfor 5 minutes before a predetermined amount of InP/ZnS core/shellQD-beads (0.2 g) was added. The resulting mixture was allowed to stir at60° C. for a period of up to 12 h and was subjected to ultrasound for5-20 min to aid particle dispersion. The process yielded a transparentgreen QD-bead ink. The InP/ZnS core/shell QD-beads used in thisexperiment were characterized by photoluminescence emission maximumPL=544 nm, FWHM=56 nm and PLQY=39%.

Example 2—Formulation of a Semiconductor QD-Bead Ink (Red Acrylate Beadsin LED Acrylate Matrix)

All work was carried out in a UV-light protected atmosphere. Apredetermined amount of initiator Irgacure 819 (6 mg) was placed in aglass vial containing a magnetic stirrer. The vial was then sealed andfilled with nitrogen. Trimethylol propane trimethacrylate (0.63 mL) wasinjected into the vial. The mixture was stirred at 250 rpm for 30 minuntil all solid was dissolved. Then, a pre-determined amount of thelauryl methacrylate (1.37 mL) was injected into the vial and the mixturewas allowed to stir for 1 hour. 200 mg of InP/ZnS core/shell QD-acrylatebeads (PLQY=47%, PI=614 nm, FWHM: 59 nm), was added to the acrylatemixture and allowed to stir under nitrogen for 1 hour to yield an ink ofthe following characteristics: EQE=48%, PI=614 nm, FWHM=58 nm.

Example 3—Formulation of a Semiconductor QD-Bead Ink (Red Acrylate Beadsin Flexible Acrylate Matrix (10% TMPTM, 2% PIB)

All work was carried out in a UV-light protected atmosphere. Apredetermined amount of Irgacure 819 (6 mg) was placed in a glass vialcontaining a magnetic stirrer. The vial was then sealed and filled withnitrogen. Trimethylol propane trimethacrylate (TMPTM) (0.22 mL) wasinjected into the vial. The mixture was stirred at 250 rpm for 30 minuntil all solid was dissolved. In parallel, in a nitrogen filled vial 20w/v % of polyisobutylene (PIB) in lauryl methacrylate (0.18 mL) wasadded to lauryl methacrylate (1.60 mL) and the mixture allowed to stirfor 15 minutes. The resulting polyisobutylene/lauryl methacrylatemixture was added to the initiator/trimethylol propane trimethacrylatemixture and allow to stir for 1 hour to yield a yellowish ink matrix.Then, 200 mg of InP/ZnS core/shell QD-acrylate beads (PLQY %=47%, PI=614nm, FWHM: 59 nm) was added to the latter matrix and allowed to stirunder nitrogen for 1 hour to yield an ink of the followingcharacteristics: EQE=48%, PI=614 nm, FWHM=58 nm.

Example 4—Fabrication of a Semiconductor QD-Bead Phosphor Sheet (GreenSilica Beads in Polystyrene/Toluene Matrix)

A doctor blade system was built as follows: a PET sheet of predetermineddimensions was cleaned with an air gun to remove dust particles. Twospacers of predetermined thickness were taped on to the PET substratemaking sure that a constant gap (15 mm) was left between the spacers.The PET substrate was then transferred into a nitrogen-filled glove box.A predetermined volume of a QD-bead ink (0.2 mL) was drop cast on theregion between the spacers on the PET substrate using a 2 mL plasticsyringe. Using a glass slide as a blade the ink was distributeduniformly between the spacers. The substrate was placed on a hot platepre-heated at 70° C. and heated for 10 min in order to remove thesolvent. The resulting film exhibited noticeable fluorescence underbright ambient light conditions. The optical properties were determinedusing a spectrofluorometer equipped with an integrating sphereaccessory: photoluminescence emission maximum PL=550 nm, FWHM=55 nm andPLQY=31%.

Example 5—Formulation of a Semiconductor QD-Bead Phosphor Sheet (RedAcrylate Beads in LED Acrylate Matrix)

Inside a nitrogen-filled glove box a predetermined volume of a QD-beadink (50 μL, EQE=48%, PI=614 nm, FWHM=58 nm) was drop cast on to a glassmould (300 μm well) and irradiated with a medium-pressure mercury lamp(45 mW/cm², 4 minutes) to yield a QDbead polymer film (EQE=45%, PI=611nm, FWHM=58 nm).

Example 6—Formulation of a Semiconductor QD-Bead Phosphor Sheet (RedAcrylate Beads in Flexible Acrylate Matrix (10% TMPTM, 2% PIB)

Inside a nitrogen-filled glove box a predetermined volume of a QD-beadink (50 μL, EQE=48%, PI=614 nm, FWHM=58 nm) was drop cast onto a glassmold (300 μm well) and irradiated with a medium-pressure mercury lamp(45 mW/cm², 4 minutes) to yield a QD bead polymer film (EQE=40%, PI=607nm, FWHM=57 nm).

The present disclosure provides QD-containing beads formulated intoprintable inks that can then be used to fabricate light-emitting sheet,layer or film materials. The methods have been developed so as to besufficiently flexible and robust to enable QDs capable of emitting lightof any desirable wavelength to be processed into light-emitting(‘phosphor’) layer materials. Such materials may be caused to emit lightof the predetermined wavelength upon irradiation with, for example, UVor blue light. The color of visible light emitted by a phosphor layercan be tuned from green to deep red depending on the size of the QDs andthe processes used to incorporate the QDs into the chemically stablebeads. The QD-containing beads may contain different sizes and/or typesof QDs, thus, for example, a QD-bead may contain QDs of one, two or moredifferent sizes and/or chemical composition. Depending on the number ofeach type of QD present, the bead will provide a particular color uponexcitation. These properties also allow for color tuning within the beadby combining different amounts of different color QDs within specificbeads. By modifying the encapsulation process novel functionalities canbe imparted to the QDs giving the option to disperse the QDs into a widerange of commercially available resins that are used for fabricatingconventional phosphor devices. In addition the size of the beads can betuned, for example from 50 nm to 0.5 mm in diameter, to control theviscosity of the resulting QD-bead/resin dispersion.

Because of the nature of the encapsulation process, not only is QDaggregation prevented thus yielding a uniform layer, but also the QDsurface is not disrupted or significantly modified so that the QDsretain their original electronic properties. In this way an appropriateselection of QDs (size, color, chemical composition, single or multipletypes etc.), bead materials and ink formulation can be made at theoutset and then used to produce a phosphor sheet to a specificspecification. QD-beads have the additional benefits of increasedprotection compared to “bare” QDs in terms of chemical attack by thesurrounding environment (e.g. air, oxygen and moisture) which increasesthe photostability of the QDs during QD-bead ink formulation andphosphor sheet formation, and also during the subsequent incorporationof the phosphor sheet into the final light emitting device to ensurethat the optical performance of the final device is as good as possible.

What is claimed is:
 1. A method of fabricating a light emitting deviceincorporating a light emitting layer comprising a plurality of lightemitting beads dispersed within a host matrix material, each of saidlight emitting beads comprising a population of semiconductornanoparticles embedded within a polymeric encapsulation medium, themethod comprising: providing a dispersion containing said light emittingbeads, an initiator, a cross-linker, a monomer, and polyisobutylene;depositing said dispersion on a surface of a diffuser sheet to form afilm thereon; annealing said film to produce said light emitting layeron the surface of the diffuser sheet; and placing the diffuser sheetover a light source such that the diffuser sheet is between the lightsource and the light emitting layer.
 2. The method recited in claim 1wherein the initiator is a photoinitiator.
 3. The method recited inclaim 2 wherein the photoinitiator isbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide.
 4. The method recitedin claim 1 wherein the cross-linker is trimethylol propanetrimethacrylate (TMPTM).
 5. The method recited in claim 1 wherein thecross-linker is divinyl benzene.
 6. The method recited in claim 1wherein the monomer is an acrylate monomer.
 7. The method recited inclaim 6 wherein the acrylate monomer is lauryl methacrylate.
 8. Themethod recited in claim 1 wherein said dispersion possesses a viscositysuitable for deposition by printing or drop casting.
 9. The methodrecited in claim 1 wherein depositing said dispersion on the surface ofthe diffuser sheet is by printing or drop casting.
 10. The methodrecited in claim 1 wherein said film has a thickness less than about 250nm.
 11. The method recited in claim 1 wherein the host matrix materialis a polystyrene resin.
 12. The method recited in claim 1 wherein thedispersion further comprises a polystyrene resin and toluene, andwherein the host matrix material is the polystyrene resin.
 13. Themethod recited in claim 1 wherein the host matrix material is amethacrylate.
 14. The method recited in claim 1 wherein annealingcomprises heating said film to a temperature of about 300° C.
 15. An inkcomposition, the ink comprising a dispersion containing: light emittingbeads; an initiator; a cross-linker; a monomer; and polyisobutylene. 16.The ink composition recited in claim 15 wherein the initiator is aphotoinitiator.
 17. The ink composition recited in claim 15 wherein thecross-linker is trimethylol propane trimethacrylate (TMPTM).
 18. The inkcomposition recited in claim 15 wherein the monomer is an acrylatemonomer.
 19. The ink composition recited in claim 18 wherein theacrylate monomer is lauryl methacrylate.
 20. The ink composition recitedin claim 15 wherein the light emitting beads comprise semiconductornanoparticles containing ions selected from groups 11, 12, 13, 15 and 16of the periodic table.